A common method used to identify problems with a patient's respiratory system is pulse oximetry. The color of the blood (i.e., the amounts of red and infrared radiation absorbed by the blood) is a function of the oxygen saturation of the heme in the blood's hemoglobin. For example, heme that is saturated with oxygen appears bright red because saturated heme is highly permeable to red light. In contrast, heme that is deoxygenated appears dark and bluish as it is less permeable to red light. A pulse oximeter measures the oxygen content of arterial blood by irradiating the blood with red and infrared radiation and determining the corresponding amounts of the red and infrared radiation that are absorbed by the heme in the blood.
A pulse oximeter sensor generally includes one or more emitters, a leadframe and a detector, all of which are located within a light impermeable housing. The leadframe, which is formed by a number of discrete conductive strips (also called traces), is used to electrically connect the emitters and detector with a pulse oximeter processing and display unit. The oximeter processing and display unit sequentially energizes the emitters and analyzes the resulting signals received from the detector to determine the oxygen content of the patient's blood.
There are a number of competing design objectives for pulse oximeter sensors. First, the lead frame must be thin enough to be flexible to conform to the body part to which it is attached, yet thick enough to conduct heat away from electrical parts, such as the LEDs and detector. Second, many applications, particularly when using the oximeter sensor with patients having sensitive skin such as premature infants and burn victims, it is desirable to have a relatively soft patient/sensor interface. Third, to protect the detector, LEDs and other electrical components from the terrestrial environment, it is important to have a seal between the light transparent lenses covering the LEDs and detector on the one hand and the light opaque sensor housing on the other that has a high degree of integrity. Fourth, the transition between the lens and the surrounding portions of the sensor body should also be relatively smooth to inhibit skin abrasion. Finally, it is desirable to have an oximeter sensor that is configured for ease of assembly. For example, it is desirable to have an oximeter sensor body that does not require an overmolding step during fabrication. Overmolding is an expensive process that requires high cost tooling and a molding machine. If the sensor manufacturer does not have a molding machine available, the various sensor components are typically shipped back to the molder for final assembly.